The invention concerns a thermopile infrared sensor array consisting of a sensor chip with a number of thermopile sensor elements constructed on a semiconductor substrate of a sensor chip and corresponding electronic components, like preamplifiers, addressing devices, in which the sensor chip is mounted on a support substrate and closed with a cap in which inlet optics is situated in the center above the sensor chip.
Infrared sensors that can be produced in Si micromachining are known in various types.
For example, a monolithic thermopile sensor array produced in Si micromachining is mentioned in A. D. Oliver, K. D. Wise (University of Michigan): “1024 element bulk micromachined thermopile IR arrays, in Sensors & Actuators 73 (1999), pp. 222-231.
A wet etching technology of the back side is used for the sensor elements, in which residual connectors between the elements of a relatively large 12×12 mm membrane are left as thermal separation pits by a somewhat complicated etching process. Overall a fairly large chip measuring 16×16 mm is obtained for 32×32 elements.
All elements of the array are converted via a multiplexer integrated on the chip into a serial analog signal and amplified by an also integrated preamplifier. This preamplifier must have at least the bandwidth from the image frequency (for example 20 Hz) and the element number (in the presented case 1024), i.e., for example >20 kHz. Since the sensor and preamplifier noise is influenced by the bandwidth of this preamplifier, fairly high noise is obtained, which shifts the detection limit (thermal resolution) to higher temperatures. Consequently, the presented array is also used for temperature measurement at high temperatures of >100° C. For lower temperatures (for example, room temperature) a sufficient signal/noise distance is not obtained.
The sensor chips achieved with the presented wet etching technology are relatively large, which leads to high manufacturing costs.
A monolithic thermopile sensor array in which the sensor elements are produced by surface micromachining with a sacrificial coating is mentioned in Kanno, T. et al. (NEC Corp.): “Uncooled focal plane array having 128×128 thermopile detector elements” in B. Andersen (editor), Infrared Technology, Proc. SPIE 2269, Vol. XX, San Diego, July 1994, pp. 450-459.
The readout technique with a CCD register permits relatively good resolution capacity but only if the sensor chip is capped in a vacuum-tight housing. Vacuum-tight housings, however, increase the sensor costs significantly and limit the applicability for cheap high-volume applications.
A monolithic thermopile sensor array produced in bulk by Si micromachining is mentioned in Schaufelbühl, A., Münch, U. (ETH Zurich): “256 pixel CMOS integrated thermoelectric infrared sensor array” in MEMS 2001, The 14th Intern Conference on Micro Mechanical Systems, Interlaken, Switzerland, Jan. 21-25, 2001, Proceedings, p. 200-203, and in Münch, U. et al.: “Thermoelectric sensor,” U.S. Pat. No. 6,040,579. A wet etching technique of the back side is again used. The forming, relatively large membrane is thermally separated by thick gold connectors between the pixels in contrast to A. D. Oliver, K. D. Wise.
A wet etching technology of the back side is used for the sensor elements. The monolithic 16×16 array chip presented by Schaufelbühl, A., Münch, U. with 7.4×12 mm is also fairly large and costly to produce. Two preamplifiers are arranged on the two sides of the chip for preamplification. It is not described how the preamplifiers are connected, but the total noise of the circuit with 256 elements is relatively high because of the high bandwidth and the image frequency remains low and only image frequencies of 1 Hz are mentioned. The power loss [sic] arranged on the two sides of the chip contribute because of their power loss to a thermal offset so that a separation of the sensor chip and preamplifiers is proposed as an alternative.
If, however, individual preamplifiers are arranged outside of the sensor chip, the space requirements and manufacturing costs are further increased.
A monolithic thermal pile sensor array in which the sensor elements are produced by surface micromachining by wet etching of the front is mentioned in Masaki Hirota et al. (Nissan Motor Company): “Thermoelectric Infrared Imaging Sensors for Automotive Applications,” Proc. of SPIE, Vol. 5359, pp. 111-125. Each sensor element contains only one thermal element with high sensitivity.
In principle, an acceptable temperature resolution is achieved with this method, in which a vacuum-tight sensor housing is proposed.
The vacuum-tight housing again stands in the way of cost-effective mass production.
A monolithic thermopile sensor array produced in bulk Si micromachining is mentioned in the HORIBA product information: “8×8 element thermopile imager,” in Tech Jam International, 26 Sep. 2002.
The 64 elements are situated on an 8×8 mm chip, each element being separated thermally by Si walls in the wet etching technology. The size of the chip related to the process leads to relatively high manufacturing costs and again stands in the way of cost-effective mass applications.
In addition to these thermopile solutions, there are other solutions for low-cost infrared arrays:
Monolithic bolometer structures for infrared sensor arrays are presented in B. E. Cole, C. J. Han (Honeywell Technology Center): “Monolithic 512×512 arrays for infrared scene projection,” Conference Transducers 95/Eurosensors, Stockholm, Sweden, 25-29 Jun. 1995, pp. 628-631 or in EP 0 869 341 A1.
The sensor elements in these infrared sensor arrays are produced by surface micromachining, in which removal of a sacrificial coating leads to thermally very well insulated sensor bridges about 2.5 μm above the Si substrate, which contains the evaluation circuit.
Such infrared bolometers with sensor bridges have since become available in many variants. Because they permit very small element dimensions, they are widespread in high-resolution infrared arrays.
In principle, despite the small sensor element dimensions, very good temperature resolutions are achieved with this method. However, the small element dimensions on the silicon surface necessarily require vacuum-tight packing of the sensor chip, which again stands in the way of cost-effective mass production.
Hybrid pyroelectric arrays with a readout circuit in silicon are presented in Q. Q. Zhang, B. P. Loss et al. (Hong Kong University): “Integrated pyroelectric array based on PCLT/P (VDF/TrFE) composite,” Sensors & Actuators 86 (2000), pp. 216-219 as well as R. Kennedy McEwen (GEC Marconi): “European Uncooled Thermal Imaging Technology,” SPIE, Vol. 3061, 1997, pp. 179-190.
Because of the high sensitivity of the sensor elements pyroelectric sensor arrays permit high thermal resolution. However, the hybrid technology increases the costs in comparison with monolithic sensor arrangements in silicon technology. In addition, pyroelectric sensors generally have the drawback that they only respond to varying objects. For thermal imaging of resting objects—which represents the normal case—continuous modulation of the radiation flux is necessary, which is generally achieved with a mechanical chopper. Additional mechanically moved parts reduce the reliability and increase the mechanical size as well as the costs of an infrared sensor array.
In the prior art cited above thermal infrared sensor arrays are proposed, which have cost drawbacks for production of infrared sensor arrays in large numbers either                because of a large-surface chip technology (A. D. Oliver, K. D. Wise, Schaufelbühl, A., Münch, U., Münch, U. et al. and Horiba product information)        a costly vacuum housing technology (Kanno, T. et al., Masaki Hirota et al., B. E. Cole, C. J. Han and Oda, Naoki or        an additional mechanical chopper assembly. The underlying task of the invention is to provide a monolithic infrared sensor array that has high thermal resolution capacity with a small chip size and can be produced cost-effectively in large numbers without demanding vacuum housing technology or mechanically moving additional parts.        